1) Field of the Invention
The invention relates to a metallized or ceramically coated, biaxially oriented polyester film with high oxygen barrier and built up from at least one base layer B and, applied to this base layer, at least one outer layer A, where this outer layer A has a defined number of elevations (the terms elevation and protrusion are used interchangeably throughout) of defined height and diameter, and where at least this outer layer A is metallized or ceramically coated. The invention also relates to the use of the film and to a process for its production.
2) Prior Art
In many foodstuff packaging applications, there is demand for a high barrier effect against gases, steam and flavors (this having the same significance as low transmission or low permeability). A well known process for producing packaging of this type consists in high-vacuum aluminum metallizing of the plastic films used. Another well known process consists in coating the films with ceramic materials (e.g. SiO.sub.x, AlO.sub.x or MgO.sub.x). Essentially, the coatings used are transparent.
The barrier effect against the substances mentioned above depends essentially on the type of polymers in the film and the quality of the barrier layers applied. Thus a very high barrier effect against gases, such as oxygen and flavors, is achieved in metallized, biaxially oriented polyester films. A barrier effect against steam is achieved in metallized, biaxially oriented polypropylene films.
The good barrier properties of metallized or ceramically coated films mean that they are used in particular for packaging foodstuffs and luxury foods, for which long storage or transport times create the risk that the packed foodstuffs become spoilt, rancid or lose flavor if there is an inadequate barrier; examples are coffee, snacks containing fats (nuts, chips, etc.) and drinks containing carbon dioxide (in pouches).
If polyester films metallized with an aluminum layer or having an applied ceramic layer are used as packaging material, they are generally a constituent of a multilayer composite film (laminate). Bags produced therefrom can be filled, for example, on a vertical tubular bag forming, filling and sealing machine (vffs). The bags are heat-sealed on their inward side (i.e. on the side facing the contents), the heat-sealable layer consisting generally of polyethylene or polypropylene. The composite film here typically has the following structure: polyester layer/aluminum or ceramic layer/adhesive layer/heat-sealable layer. If the laminate thickness is from about 50 to 150 .mu.m, the thickness of the metal or ceramic layer is only from 10 to 80 nm. Even this very thin layer is sufficiently effective to achieve adequate protection from light and very good barrier properties.
The oxygen barrier or the oxygen transmission is generally measured not on the laminate or the packaging itself, but on the metallized polyester film. To ensure good quality of the foodstuffs or luxury foods even after relatively long storage times, the oxygen transmission (identical with permeability) of the metallized film may not be greater than 2 cm.sup.3 /m.sup.2 bar d, (i.e. not greater than 2 cubic centimeters of oxygen per square meter of film at a pressure of 1 bar per day) but in particular not greater than 1 cm.sup.3 /m.sup.2 bar d. In future, the demands of the packaging industry will head toward still higher barriers, with attempts to achieve permeability values of significantly less than 1.0 cm.sup.3 /m.sup.2 bar d for metallized or ceramically coated films.
In the prior art, there is neither sufficient knowledge of the detailed basis for the barrier effect of metallized or ceramically coated polyester films nor of how this may be decisively improved. Variables which are clearly important are the surface of the substrate and the type of substrate polymer and its morphology. It is generally assumed that smooth substrate surfaces result in better barrier properties.
In this connection, Weiss et al., in "Thin Solids Films" 204 (1991), pp. 203-216, studied the influence of the surface roughness of a substrate layer on its permeability. For this, polyester films were coated with lacquer which contained various concentrations of titanium dioxide particles. In his experiments, the concentration of titanium dioxide particles in the lacquer was varied from 2 to 20% by weight. Using this method, the surface roughness of the coated substrate surface could be varied from 43 nm (unlacquered and lacquered film, without titanium dioxide) to 124 nm. In his experiments, increasing roughness (increasing proportion of TiO.sub.2) of the lacquered surface resulted in markedly higher oxygen transmissions after metallizing with aluminum. However, the largest step increase in oxygen transmission was seen when the lacquered film (0% by weight TiO.sub.2) was compared with the unlacquered film, although the surface roughness of the substrate was the same in both cases. The lacquering alone of the film gave a deterioration in the barrier from about 0.43 cm.sup.3 /m.sup.2 d bar (plain film) to about 19 cm.sup.3 /m.sup.2 d bar (lacquered film). A further disadvantage of his studies was that the aluminum layer was applied using a laboratory evaporator. When compared with an industrial metallizer, this method achieves essentially low permeability values, and the influence of the substrate surface cannot be seen clearly.
Other detailed results of studies on the influence of the substrate surface of polyester films on their barrier properties can be found in the dissertation by H. Utz (Technische Universitat Munchen 1995: "Barriereeigenschaften aluminiumbedampfter Kunststoffolien" [Barrier properties of aluminum-metallized plastic films]).
According to the studies by Utz (p. 66 ff.), there is no direct correlation between the surface roughness (average roughness height R.sub.a) of the PET film and its oxygen barrier. For example, the film for video applications which has an average roughness height of R.sub.a =22 nm, is particularly smooth and has an oxygen transmission of 1.3 cm.sup.3 /m.sup.2 bar d compared with a much rougher PET II (R.sub.a =220 nm) film having an oxygen barrier of 1.2 cm.sup.3 /m.sup.2 bar d.
EP-A-0 124 291 describes a single-layer biaxially oriented polyester film for magnetic recording tape which has the following surface property parameters
a) the average roughness R.sub.a is from 1 to 16 nm, PA1 b) the coefficient of friction .mu.k is from 0.01 to 0.20 and PA1 c) the following relationship exists between R.sub.a and .mu.k EQU 0.1.ltoreq.10 * R.sub.a +.mu.k.ltoreq.0.31. PA1 a) from 0.05 to 1.0% by weight of .theta.-alumina (theta aluminum) having an average particle diameter in the range from 0.02 to 0.3 .mu.m, and PA1 b) from 0.01 to 1.5% by weight of inert particles of a type other than .theta.-alumina and having an average particle diameter in the range from 0.1 to 1.5 .mu.m, these particles being larger than the .theta.-alumina particles. PA1 a) a coated surface A, which is free from particles and PA1 b) a second layer containing particles and having relatively rough surface and composed of PA1 a) has an average roughness R.sub.a (peak-valley value) of not more than 5 nm (60 nm), PA1 b) the number of protrusions having a height of from 0.27 to 0.54 .mu.m is from 0 to 0.2 per mm.sup.2 and PA1 c) is free from protrusions having a height greater than 0.54 .mu.m. PA1 i) a surface having a surface roughness R.sub.a of from 5 to 40 nm and a large number of depressions and a large number of protrusions which are arranged in a particular arrangement or PA1 ii) a surface which has protrusions formed on a level area and which is covered by a layer C, which consists of a lubricant and has a surface roughness R.sub.a of from 5 to 40 nm. PA1 a) contains inorganic particles having an average primary particle size D in the range from 1 to 100 nm and satisfying the equation D&lt;T&lt;200D, where T is the thickness of the layer A; PA1 b) contains particles B having an average primary particle size D1 in the range from 0.3 to 2 .mu.m, where the primary particle size distribution has a coefficient of variation of not more than 0.6; and PA1 c) the average primary particle size D of the particles A is smaller than the average primary particle size D1 of the particles B. PA1 A.sub.h =1.4; B.sub.h =2.5 EQU log N/mm.sup.2.ltoreq.A.sub.d -B.sub.d (log d/.mu.m), 0.01 .mu.m&lt;d&lt;10 .mu.m (2) PA1 A.sub.d =3.4; B.sub.d =2.4 PA1 a) a good oxygen barrier (low permeability values) according to the present invention (FIG. 1a) and PA1 b) a low oxygen barrier (high permeability values) (FIG. 1b). PA1 only a few pigment particles are recorded and PA1 the pigment particles are encountered randomly.
These properties are created by using TiO.sub.2 particles (anatase) or TiO.sub.2 and CaCO.sub.3 particles in a proportion by weight of, respectively, from 0.1 to 0.5% and from 0.1 to 0.3%. The diameter of the TiO.sub.2 particles is from 0.1 to 0.5 .mu.m. The surface of this film is formed by a large number of elevations/protrusions ("the excellent slipperiness of the polyester film of this invention is simultaneously achieved by the presence of the many very minute protrusions") and these obey a distribution such that the graph described by the following relationship EQU log y=-8.0 x+4.34, y&gt;10
is not intersected. In this equation, x (.mu.m) is a height above a standard level and y is the number of elevations (number/mm.sup.2) if the elevations are sectioned at a height of x. The distribution of the elevations is determined using standard equipment for measuring roughness.
EP-A-0 490 665 A1 describes a single-layer biaxially oriented polyester film for magnetic recording tape; the film contains
The surface of this film is formed by a large number of elevations/protrusions which are described by the relationship EQU -11.4 x+4&lt;log y&lt;-10.0 x+5 y&gt;30, x&gt;0.05 .mu.m.
In this equation, x (.mu.m) is a height above a standard level and y is the number of elevations (number/mm.sup.2) if the elevations are sectioned at a height of x. The distribution of the elevations is measured as in EP-A-0 124 291.
The prior art also discloses films which have different surfaces (dual surface). These films are suitable in particular for magnetic recording media and essentially have different topographies (e.g. surface A smooth, surface B rough).
DE-A-16 94 404 describes a layered material having more than one layer of an oriented crystallizable thermoplastic film and in which at least one of the outer layers contains an additive. The additives are customary inert inorganic or organic particles, and in the case of inert particles such as SiO.sub.2, are added to the outer layers in concentrations of from 1 to 25% by weight, the particle size being from 2 to 20 .mu.m. The layered materials may, for example, be metallized with aluminum for decorative purposes or used for magnetic tape.
DE-A-22 30 970 describes a magnetic recording medium which is composed of a biaxially oriented polyester film and a thin magnetic metallic layer on the surface A of the polyester film. The film comprises
i) is at least 4 .mu.m thick or PA2 ii) makes up at least 50% of the thickness of the entire film layer; and PA2 i) at least 1% of individual particles of a particular polymer A and PA2 ii) at least 1% of individual particles of a particular polymer B.
EP-B-0 061 769 describes a magnetic recording medium which is formulated from a biaxially oriented polyester film and a thin magnetic metallic layer on the surface A of the polyester film. If desired, there is also a lubricant layer on the other surface B of the polyester film. Features of the film are that the coated surface A
EP-B-0 088 635 describes a coextruded biaxially oriented polyester film having at least two layers, of which a layer A consists of thermoplastic resin and a layer B comprises thermoplastic resin and fine particles. The surface roughness R.sub.a of the outer layer on the layer A in the film is less than 5 nm and the outer surface of the layer B is either
According to Example 1 of this text, the surface layer A is "substantially free of internal particles formed by reaction of the catalyst residue . . . (page 7)". A disadvantage of this film surface is that it blocks with itself and with certain other surfaces (e.g. rubber rolls). The film cannot be processed cost-effectively and, in particular during vacuum metallizing, it tends to tear because of its high blocking tendency, and this can cause great cost problems. The film is unsuitable for the purposes of the object to be achieved.
EP-B-0 502 745 describes a coextruded, biaxially oriented polyester film having at least three layers, of which an outer layer A
It is also known that the oxygen barrier can be improved by selecting particular polymers for the film serving as substrate (Schricker, G.: Metallisierte Kunststoffolien fur hoherwertige Verpackungen [Metallized plastic films for high-quality packaging] in: ICI 5th International Metallizing Symposium 1986, Cannes). Polyesters, for example, are particularly suitable, specifically those made from ethylene glycol and terephthalic acid or from ethylene glycol, terephthalic acid and naphthalene-2,6-dicarboxylic acid. Besides these, polyamides, ethylene-vinyl alcohol copolymers (EVOH) and polyvinylidene chloride may also be used with practical advantage. Thus, for example, U.S. Pat. No. 5,506,014 describes a copolyester made from: (a) from 45 to 85 mol % of terephthalic acid; (b) from 10 to 40 mol % of naphthalenedicarboxylic acid; (c) from 5 to 15 mol % of a dicarboxylic acid having from 2 to 8 carbon atoms; and (d) ethylene glycol; (the molar percentages are based on the total proportion of dicarboxylic acids). This polyester is claimed to have better barrier properties against gases. It is used, inter alia, for producing bottles or containers and films of various thicknesses. A disadvantage of the raw materials mentioned is that they are significantly more expensive than polyethylene terephthalate (PET) or are unsuitable and/or not officially permitted for use in the packaging of foodstuffs.
It is an object of the present invention to provide a metallized or ceramically coated, coextruded, biaxially oriented polyester film which exhibits a high oxygen barrier. Less than 0.5 cm.sup.3 of oxygen per square meter and per day should diffuse through the film when it is subjected to an air pressure of 1 bar. In its other properties, the film should be at least equivalent to the known packing films of this type. Moreover it should be simple and economic to produce and should process well on conventional machinery (i.e. not block, for example). It was also an object to improve the optical properties (i.e. in particular the gloss) of the metallized or ceramically coated film.